1. Field of the Invention
The present invention relates to a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
An exposure apparatus is employed to fabricate micropatterned semiconductor devices using photolithography. The exposure apparatus projects and transfers a pattern formed on a reticle (mask) onto a substrate such as a wafer via a projection optical system. In recent years, an exposure apparatus of the step & scan scheme (scanner) has become the mainstream in place of an exposure apparatus of the step & repeat scheme (stepper).
An exposure apparatus of each of these types performs correction in which it measures the surface position of a substrate at a predetermined position defined on it using a surface shape (surface position) measurement means of the light oblique incidence system before exposure (or during exposure), and aligns the substrate surface with an optimum imaging position in exposing the substrate at the predetermined position. This correction is of prime importance especially for a scanner which measures not only the height (focus) of the substrate surface position in the longitudinal direction of the exposure slit (that is, a direction perpendicular to the scanning direction) but also the surface tilt with respect to that direction. U.S. Pat. No. 4,340,306 and U.S. Patent Application Publication No. 2007/0086013 propose details of such techniques.
The so-called white light interference signal scheme is widely used to measure the surface shape of a substrate in an exposure apparatus. In the white light interference signal scheme, white light emitted by a light source is split into two light beams to guide one light beam to a test surface (substrate) and the other light beam to a reference surface. Measurement light reflected by the test surface and reference light reflected by the reference surface are combined, and an interference pattern (interference signal) formed by the interference between the measurement light and the reference light is detected. At this time, as the height position (the position in the Z-axis direction) of the test surface changes, the optical path difference (OPD) between the reference light and the measurement light changes, and the interference signal between the reference light and the measurement light, in turn, changes. This makes it possible to obtain the surface shape of the test surface from the change in interference signal. Note that the conventional white light interference signal scheme requires detecting an interference signal while driving (scanning) the test surface in the height direction for each measurement point. Thus, in this scheme, it takes a relatively long time to measure the three-dimensional shape of the test surface.
U.S. Pat. No. 4,340,306 discloses a technique of shortening the measurement time by two-dimensionally arraying pixels which detect interference signals to widen a measurement region which can be measured by one scanning operation. However, when pixels are arrayed two-dimensionally, and the test surface is larger than the two-dimensional measurement region, it is necessary to divide the test surface into a plurality of regions, and scan it for each of the plurality of regions. Thus, in this technique, it takes a measurement time equal to the product of the number of division and the time taken for each scanning. Although it is possible to array pixels so that the test surface can be measured at once, both the size of an optical system and the number of pixels in a measurement apparatus increase in this case, and this poses problems associated with the cost and arrangement. Also, when a reduction optical system is applied to an optical system in a measurement apparatus for its downsizing, the pixel resolution decreases, and this deteriorates measurement accuracy.
U.S. Patent Application Publication No. 2007/0086013 discloses a technique of measuring the height position of the test surface by placing a spectroscopic device on the incident side of an image sensing device, and detecting the interference intensity for each wavelength on the image sensing device to obtain an interference signal. Since the technique disclosed in U.S. Patent Application Publication No. 2007/0086013 does not require scanning of the test surface in the height direction, it can measure the test surface in a short period of time. However, to obtain high measurement accuracy, it is necessary to improve wavelength resolution. To meet this requirement, a high-precision spectroscopic device and a high-pixel-density image sensing device are necessary, and this poses problems associated with the cost.